This invention relates to an ink for use in ink-jet printing, and more particularly to a white ink composition suitable for ink-jet printing of colored articles or transparent articles made of particularly wood, metal, such as iron and steel in shapes of plates, tubes, etc., plastics, glass, porcelain, leather, surface-treated cardboard, etc. and to use of the white ink composion in ink-jet printing of colored articles, particularly black or dark articles.
Before printing of desired letters on a print article, the ink for use in the ink-jet printer is converted to fine particles, when injected from a nozzle, by vibration given to the nozzle, and the thus obtained fine ink particles are electrically charged according to electrical letter signals, and undergo deviation in direction according to the electrical charges during passage between deviation electrodes under a given potential. Thus, in the ink-jet printer, the ink must have such functions that the ink is stably converted to fine particles, and forms desired printing on an article according to an electrical letter signal. Thus, the basic requirements for such an ink composition for use at a temperature of 5.degree. to 40.degree. C. are as follows:
Viscosity: 1-15 cp PA0 Surface tension: 20-60 dyn/cm PA0 Specific resistance: less than 2,000 .OMEGA..cm PA0 Specific gravity: 1-3
Further, the ink must be stable and homogeneous for a prolonged time, and must be able to print an article with the necessarily sufficient precision and concentration and rapid drying without clogging the nozzle or filter.
Heretofore, an aqueous ink using water as the main dispersant and an organic ink using an organic solvent as the main dispersant have been available for ink-jet printing, and both inks use a dye as a coloring material soluble in the respective solvents, and are directed to printing white or similarly colored articles. When a print article is heavily colored, the print itself is hidden by the proper color of the print article, and cannot be recognized.
An ink for ink-jet printing, which is applicable to metal, glass, rubber, etc. and uses an organic solvent as the main component is disclosed, for example, in Japanese Patent Application Kokai (Laid-open) Nos. 46-54147, 49-50935, 52-96106, 52-138208 and 55-50073, and U.S. Pat. No. 4,248,636, where the coloring material is limited to black or colored dye, and thus is not applicable to printing of black or dark articles.
Printing of black or dark articles are well known in the technical field of paint and ink, where mainly white pigments having a high hiding power, such as titanium white is used as the coloring material. However, the paint generally has too high a viscosity, whereas the ink generally has such a low viscosity as to pass through a felt like a felt pen ink, and the pigment is liable to settle down when the ink is left standing for a prolonged time. In this case, no consideration is given to the specific resistance.
Recently, preparation of ink for ink-jet printing having the necessary requirements for conversion to fine particles according to electrical letter signals has been tried on the basis of the paint and ink [Wire Journal (April, 1980) pages 84-87]. However, the most difficult problem encountered in using an ink for ink-jet printing using, for example titanium oxide as a white pigment, is a long-term operation. Such printing has the following two problem areas:
(1) Maintaining uniform pigment dispersion and suspension in the ink, and
(2) Elimination of pigment accumulation and blockage of system filters.
To solve the first problem area, continuous mechanical stirring of ink is necessary or a tube with a smaller diameter must be used in an ink feed system to increase an ink flow rate, and to solve the second problem area, a special filter must be used to remove oversized buildup pigments. Anyway, additional means are required for solving these problems, and there is still another problem in long-term stability of ink even if these means are used.
Addition of salts to the ink has been tried to reduce the specific resistance and also reducing of the viscosity of ink has been also tried to facilitate conversion into fine particles. However, the dispersed pigment particles are liable to be coagulated into coarser particles and to undergo settling or cause clogging of nozzle and filter.
That is, an appropriate white ink composition for ink-jet printing of black or dark articles with a practical satisfaction has not been available yet, including aqueous and organic inks.